MEXICO CITY  Marijuana is now the biggest source of income for Mexico's drug cartels and the U.S. is committed to cracking down harder on traffickers, U.S. drug czar John Walters said Thursday.

"We're trying to increase the force with which we're attacking this problem," Walters said in a telephone interview with The Associated Press. "This is a focus because of the overlooked importance marijuana has in the violence."

Walters made the comments following a meeting with Mexican officials who want the U.S. to prosecute marijuana cases more zealously to reduce the amount of cash gangs can spend on guns.

Mexican Attorney General Eduardo Medina Mora discussed the subject with Walters and U.S. federal prosecutors from the border region Thursday during a meeting in the Baja California resort of Los Cabos.

Walters said the U.S. government is seeking additional resources to prosecute traffickers of marijuana, which now earns cartels about $8.5 billion or about 61 percent of their annual estimated income of $13.8 billion. Cocaine sales earn the cartels about $3.9 billion, and methamphetamine about $1 billion, he said.

"While the criminal organizations that are a threat to both of our countries make a lot of money off of heroin and cocaine and methamphetamine, the vast majority of their money to buy guns, bribe, corrupt and destroy lives is from marijuana," said Walters, head of the White House Office of National Drug Control Policy.

Mexican officials have complained that U.S. prosecutors often release small or mid-level traffickers caught with a few dozen pounds of marijuana.

But Walters said there is no weight threshold for charges, and a desire to help end the bloody shootouts, assassinations and drug battles that have plagued Mexico in recent months could move U.S. prosecutors to act more zealously.

"There are a lot of reasons when you have a case that gets declined," Walter said. "Maybe the evidence isn't quite as strong as it seemed .... sometimes people make a judgment call, and I think it's fair to say that if you had a broader look, maybe you wouldn't make the same call."

He added that the U.S. is "looking at additional ways in which we can have a stronger prosecutorial response," including requests for more funding and personnel.

Since taking office in December 2006, Mexican President Felipe Calderon has sent thousands of soldiers and police to violence-plagued states to combat drug gangs.

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.